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heated for about 2 hours at 110-120, treated with dilute hydrochloric acid,
and the silica filtered off, washed, dried, ignited and weighed.

In the hydrochloric acid, the alumina, sodium chloride, etc., may be

(c) FREE ALKALIES. To 100 c.c. of the solution (= 10 grams of sub-
stance) are added, in a thin stream and with con/taut shaking, 100 c.c. of
10% barium chloride solution, the liquid being made up to 250 c.c., shaken
and filtered through a dry paper. The first 20-30 c.c. of the filtrate are
rejected and in 100 c.c. of the remainder ( ^4 grams of substance1) the free
alkali is titrated with N/io-hydrochloric acid in presence of phenol-
phthalein : i c.c. N/io-acid  0-004 gram NaOH.

Potassium silicate is analysed similarly.

In commercial sodium silicate of good quality the ratio of NaaO to SiO., is'
about 3:1, wliile free alkali is I'ouud only in small quantity (less than 0-5%)
and the extraneous impurities do not total 2%.


NaaSnOs + 3Ha()  267

Hard white crystals or crystalline masses, somewhat efflorescent, soluble
in water (in the air the solution becomes cloudy owing to formation of
oxide of tin), insoluble in alcohol. Its commoner impurities arc; sodium
carbonate, hydroxide, chloride and sulphate, ami iron. Double salts, con-
sisting of sodium stannate and arsonate, or sodium tungstate and staimatc,
are also sold. Analysis includes the following :

1.  Solubility."i gram, with io c.c. of water, should give a clear or
barely opalescent solution,

2.  Sodium Carbonate and Hydroxide. -A lew 1'ragniruls, dropped
into dilute hydrochloric acid, should give no effervescence (ciD'hotMk], and
if the. substance is dissolved in a little water and (hen shaken with absolute
alcohol, the liquid should not: have an alkaline rear!ion (liydrwidc).

3.  Chlorides, Sulphate, Iron.    ii grams arc dissolved in  io e,e. of
water, acidified with nitric acid and filtered :   the nitrate is tested with
silver nitrate (chlorides}, barium chloride (sulphates} and ammonium thio-
eyanate (iron}.

4.  Sodium Arsenatc.i gram is heated in a porcelain dish with 5 c.c.
of nitric acid (i ; i) on a steam-bath and evaporated to dryness, the residue
being taken up in water and a few drops of nitric acid and the liquid fil-
tered.   To the nitrate is added an excess of silver nitrate, tin; liquid again
filtered if necessary and very dilute, ammonia poured carefully on to the
clear filtrate : in presence of arsenate, a reddish ring forms at the scone of
contact of the two liquids.

5.  Sodium Tungstate. i a grains are dissolved in io c.c. of water
and the liquid filtered and treated with excess of hydrochloric acid :   in
presence of tuiigstate a yellowish, white gelatinous precipitate is formed which
becomes blue when heated gently with a very small quantity of /.inc. dust.

6.  Determination of the Alkali.- -10 grains are dissolved in waterthe filtrate then treated as described.CHLOKINE.   50 c.c. of this solution (= i gram of salt) are titrated